This invention concerns an improved screening process and apparatus for separating coarse particles from a suspension of a particulate solid material in a liquid.
Coarse particles are conveniently separated from a suspension of a particulate solid material in a liquid by means of a screen or sieve which generally consists of a perforated plate, a grid or grating or a mesh material. The mesh may be of metallic or plastics material. When the coarse particles to be separated are relatively small, i.e. in the size range from about 0.040 mm to 1 mm, a wire mesh material is the preferred separating medium.
Especially when the suspension contains a relatively large proportion of solid particles of relatively coarse size it is generally necessary to agitate the screen, for example by applying vibration thereto by means of an electric motor connected to the screen through a suitable reciprocating linkage. Such means increase the cost of the screening operation and lead to more rapid wear and consequent tearing of the screen or sieve. The screening apparatus is also noisy in operation.
Screening processes generally also suffer from the disadvantage that constant supervision is necessary because there is a tendency for the screens to blind or become blocked with solid particles which in turn causes unscreened suspension to overflow from the screens by the route intended for the oversize particles with consequent waste of fine particles.
It is known to provide screening apparatus in the form of a trommel of screening material rotating about substantially horizontal axis, which was intended to overcome the disadvantages associated with a vibrating screening device. Such an apparatus is describd in British Patent No. 2053736. However the apparatus, although quieter and less susceptible to wear and fracture of the separating medium, suffered from blinding of the screen by coarse particles.
A screening apparatus having coaxial conical or frusto-conical screens which are rotatable at different angular velocities and in different directions about a vertical axis is known from European Patent Specification No. 0278124. According to one embodiment described in that Patent specification a mixture of fine and coarse particles is dumped near the apex of an upper conical screen rotating with its apex upward. Particles retained on the screen move to the periphery of this screen and are transferred to a lower frusto-conical screen rotating with its imaginary apex downwards and in the opposite direction. Fine particles pass through the two rotating screening surfaces while the coarse particles are eventually discharged over the peripheral rim of the lower frusto-conical screen. The apparatus is designed for dry separations, for example for separating dust or ultrafine particles from cattle feed pellets [column 1, lines 46-49].
A screening apparatus having at least one circular screening member rotating in its own plane about a central axis is known from British Patent specification No. 2088750. However in that case the screening member consists of an array of radially extending bars or rods which together make up a grating and the apparatus is for dry separation of relatively coarse solid material such as coal.
Rotary screening apparatus is also known from U.S. Pat. No. 3,061,095, which shows a screen formed as a shallow cone having a vertical rim around its periphery. German Patent 41604 has rotary screens which are planar and provided with an annular rim. In each of the above the peripheral speed of the screens needs to be such as to give a centrifugal force capable of driving the coarse particles of the suspension upwardly and over the edge of the rim of the screen.
German 0LS2018677 discloses a rotary screen whose surface extends upwardly towards its periphery. Similarly, the peripheral speed of the screen needs to be sufficiently high to drive the coarse particles upwardly across the surface of the screen and over its edge.
In the above three patents the rotational speed necessary to drive the coarse particles of the suspension over the rims of the screens is found to rapidly blind the screens and an optimum speed which would not cause blinding of the screens is insufficient to centrifuge the coarse particles over the rims.
According to the invention there is provided a process for continuously separating relatively coarse particles from a suspension of a mixture of particles in a liquid wherein the suspension is caused to impinge upon a surface of a screening medium which is rotating in its own plane about a vertical axis so that the relatively coarse particles are retained on the surface of the screening medium and are caused to move radially outwardly towards the periphery of the screening medium while the liquid containing relatively fine particles passes through the screening medium and wherein the screening medium comprises a pervious material which is substantially planar and whose periphery lies in the same plane as the surface of the screening medium so that the relatively coarse particles are caused to pass unencumbered outwardly and downwardly from the periphery of the screening medium.
The suspension may be caused to impinge on the surface of two or more similar screening media in series, and these screening media may be disposed one above the other. The screening medium or media may be rotated at changing speed cycles during the screening operation.
It is preferred that the suspension containing the mixture of particles is caused to impinge on the surface of the rotating screen medium at or near the centre of rotation of the screening medium.
The suspension containing the mixture of particles may be prevented from passing through part of the screening medium or media by reason of an impervious annular band adjacent the periphery or peripheries of the screening medium or media.
In a preferred form the process is performed continuously and the coarse particles of the suspension are caused to travel radially across the screening medium without causing obturation of the passageways therein.
The suspension containing the mixture of the particles may be assisted in its passage through the screening medium or media by vibration of said screening medium or media in addition to its being rotated.
The invention further includes apparatus for continuously separating relatively coarse particles from a suspension of a mixture of particles in a liquid including a screening medium rotatable in its own plane about a vertical axis and capable of causing the relatively coarse particles to move radially outwards to the periphery of the screening medium, characterised in that the screening medium comprises a finely woven mesh screen or a perforated plate whose periphery lies on the same plane as the surface of the screening medium and is free of any encumbrance which would prevent the relatively coarse particles passing outwardly and downwardly from the periphery of the screening medium.
Two or more similar screening media may be provided in which case they are disposed one above the other.
Preferably there is provided means to cause the mixture of particles to impinge on the surface of the rotating screen medium or media at or near the centre of rotation of the screening medium or media.
Means may also be provided to collect and discharge coarse particles which overflow from the periphery of the sceening medium and there may be further provided means for vibrating the screen medium or media in addition to the means for rotating said medium or media.
The screening medium or media may be provided with an impervious or imperforate annular band adjacent its periphery or their peripheries.
The efficiency of the screening device according the invention depends on the peripheral speed of the screen. When the screen is stationary it rapidly blinds with coarse particles and the percentage by volume of the feed suspension which passes through the screen rapidly decreases. When the screen is rotated at relatively low speeds, i.e. with a peripheral speed of up to about 2 ms .sup.-1, most of the feed suspension is flung off the screen by centrifugal action, due to the at least partial blinding of the screen.
Surprisingly, when the rotational speed of the screen is within a critical range from around 2 ms.sup.-1 to 10 ms.sup.-1 for many typical feed suspensions, there is a dramatic decrease in the amount of suspension which is flung off, with the result that most of the feed suspension then flows through the screen.
In experiments it was found that, at an optimum peripheral speed for a particular feed suspension, up to 95% by volume of the feed suspension passed through the screen. In view of the relatively high viscosity of the feed suspension this is a most surprising result.
In comparison with the conventional type of vibratory screen, the rotary screen is found to have about five times the capacity on the basis of flow rate per unit area.
Also the wear and tear and the noise of conventional vibratory screens as well as the tendency for the screen cloth to rupture through fatigue are unacceptable and the use of a rotary screen overcomes these further disadvantages.